Even when there is no incident light, optical sensors usually have a background current (also referred to as the “dark current”), due to thermal effects. In order to accurately measure light incident on pixel sensors, it is necessary to know this dark current. A common solution to this problem is to include diodes or pixels in the optical sensor which are not exposed to light, to act as reference diodes/pixels to enable the determination of this dark current. These diodes/pixels are referred to as “dark diodes” or “dark pixels”. The correct light level in the detecting sensors can then be determined by subtracting the dark current from the current generated by light receiving sensors.
A typical arrangement for dark pixels is to have a light shield covering a subset of the pixels in a light sensor array. A typical arrangement is shown in FIG. 1, which shows a sensor array (101), with row (102) and column (103) control logic, and a metal plate (104) placed over a section of the sensors so as to shield the dark pixels. The dark current can then be determined.
A problem which may occur with such an arrangement however is that stresses tend to develop in any large piece of metal, which may result in deformation and consequent problems such as delamination. It is common practice for such metal shields to have slots inserted to allow relief of these stresses. CMOS design rules for example require such slots at fixed intervals. Clearly, slots or openings in a light shield will cause problems in that the light shield will no longer fulfil its function.
The problem is addressed in U.S. Pat. No. 6,278,169, in which a second metal layer is placed beneath the light shield. This second layer also has slots, which are located such that they are staggered with respect to the slots in the upper layer. This arrangement is illustrated in FIG. 2, which shows upper layer (201) and lower layer (202) with respective slots (203, 204). However light can still be reflected between the layers (205) and hence reach the light sensors below, rendering the dark diode or pixels not truly dark. In U.S. Pat. No. 6,278,169 this problem is addressed by the use of an antireflective coating (206) applied to the metal layers.
However, the present inventors have appreciated that this solution is not completely effective against light penetration to the sensors and additionally is complex to manufacture and adds cost to the production of the sensor devices.